Arctic Circle, parallel of latitude on the surface of the Earth at 66°30’ N. It marks the southern limit of the area in which the Sun does not rise on the winter solstice (usually December 21 or 22) or set on the summer solstice (usually June 21 or 22). Periods of continuous day or night increase from one day at the Arctic Circle to six months at the North Pole (see Solstice).